In the conventional multi-chip package, a plurality of chips are electrically connected to a substrate by means of flip-chip bumping and wire-bonding technologies, the chips includes a bumped chip and a wire-bonding chip that are stacked back-to-back. A multi-chip stacked package 100 is disclosed in U.S. Pat. No. 6,157,080. Referring to FIG. 1, the multi-chip stacked package mainly includes a substrate 110, a bumped chip120, a wire-bonding chip 130, a package body 160 and a plurality of solder balls 170. The substrate 110 has a plurality of bumping pads 111 and a plurality of wire-bonding pads 112 formed on the top surface of the substrate 110. The bumped chip120 is flip-chip mounted to the bumping pads 111 of the substrate 110 via a plurality of bumps 140. The wire-bonding chip 130 is attached to the back surface of the bumped chip 120. And a plurality of bonding pads of the wire-bonding chip 130 are further electrically connected to the wire-bonding pads 112 of the substrate 110 by a plurality of bonding wires 150. After electrical connection, the package body 160 is formed on the top surface of the substrate 110 in order to encapsulate the bumped chip 120 the wire-bonding chip 130, the bumps 140 and the bonding wires 150. The solder balls 170 are placed on the bottom surface of the substrate 110. However, the oxidization-resistance structures of the bumping pads 111 and the wire-bonding pads 112 on the substrate 110 are not shown. Normally a reflow step is performed after flip-chip assembly prior to wire bonding. When the wire-bonding pads 112 or the bumping pads 111 are oxidized during packaging processes, the electrical connection of the bonding wires 150 or the bumps 140 are hard to bond to the wire-bonding pads 112 or the bumping pads 111, as the results, the productivity and yield will decrease.
Another conventional structure of contact pad of the package substrate is disclosed in R.O.C. Taiwan Patent No. 515,061 entitled “Ni/Au electroplating process and structure for electrical contact pads of chip package substrate”. In order to protect the contact pads of the substrate, the exposed surfaces of the contact pads are plated with a Ni/Au layer so as to protect the contact pads from oxidation during packaging processes. However, the contact pads with plated Ni/Au layer are only for the connection of the Au bonding wires. However, when the solder bumps of the bumped chip are bonded to the Ni/Au layer, the embrittlement of Au will easily happen.
In the foregoing package substrate, when a plurality of bumping pads and a plurality of wire-bonding pads are simultaneously formed on the same surface of the substrate for assembling a back-to-back multi-chip package. The bumping pads are also electroplated with a Ni/Au layer, the solder bumps and the Ni/Au layer on the bumping pads can trigger Au embrittlement during the solder bumps reflow process. Initially, Au element of the Ni/Au layer on the outermost surface will migrate into the solder bumps extremely fast. After complete reaction of the Au layer, then the Ni layer begins to react with the solder bumps to form a pin-shaped Ni3Sn4 intermetallic layer, and the Au element enters the solder bumps to form a (Au1−XNiX)Sn4 intermetallic layer at the bonding interface after reflow. The (Au1−XNiX)Sn4 intermetallic layer is unstable so that it will accumulate at the bonding interface of the bumps of bumped chip and bonding pads of the substrate. The (Au1−XNiX)Sn4 intermetallic layer will expand gradually until the whole bonding interface is covered. The (Au1−XNiX)Sn4 intermetallic layer is fragile so that the bonding interface will easily crack, that is so-called Au embrittlement. Especially, the area of the bumping pads is smaller than the area of the conventional solder ball pads, since the bumping pads are used to mount the bumps of the bumped chip, not solder balls for BGA packages. The Au concentration in the bumps will be higher than that in the conventional solder balls where the Ni/Au electroplated layer is supposed to be the same thickness on the bumping pads and on the solder ball pads. Therefore, the reliability of the packaging body is seriously affected due to too much (Au1−XNiX)Sn4 intermetallic in the bumps.